atomic: fix atomic_add_unless() fallback's return value

According to the kernel documentation:
  Returns non-zero if @v was not @u, and zero otherwise.

Fixes: 63fc5718 ("atomic: add atomic_add_unless()")
Closes: mesa/drm#17
Signed-off-by: David Shao <davshao@gmail.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>

[Eric: fix its callers to maintain current behaviour]
Signed-off-by: Eric Engestrom <eric@engestrom.ch>
4 jobs for atomic_add_unless in 12 minutes and 8 seconds (queued for 2 minutes and 43 seconds)
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passed #660066
latest-autotools

00:03:46

passed #660065
latest-meson

00:03:09

passed #660068
oldest-autotools

00:03:17

passed #660067
oldest-meson

00:02:25